The present invention accelerates ions into a circular beam and may keep them there indefinitely by inputting only that much energy that is necessary to mitigate Coulombic scattering and ionising collisions. This is an efficiency gain over existing ion-beam forming methods potentially sufficient to enable viable beam-fusion net energy gains not otherwise possible with other beam methods. The present invention also avoids the need for ion-producing modules or elements, but generates ions within its normal operation by accelerating background ions and electrons and then using those to form further ions.
It is similar to the cyclotron invention of Ernest Lawrence (U.S. Pat. No. 1,948,384). In the cyclotron, a magnetic field is applied across a space in which ions are accelerated in a plane orthogonal to the magnetic field by synchronous radio frequency (RF) switching of circumferential electric fields between electrodes. Work is done on ions as they pass, and thus accelerate, across the space between the electrodes by the electric fields.
In the cyclotron, the centripetal acceleration of an ion is balanced to the RF electric field frequency. The magnetic force Bqwr (B=magnetic field, w=frequency, r=ion's distance to centre of rotation, q=ion's charge) balances the radial force arising from the centripetal acceleration, mw2r (m=ion mass). Therefore the ions rotation is described by mw2r=Bqwr.
Solutions exist independently of r providing mw=Bq. Consequently, ions may be incrementally accelerated by repeatedly passing across circumferential electric fields. Each time the ion gains kinetic energy so its radius freely increases. It does so until it reaches a given design radius and is then removed from the device by various means. Many such ions undertake the same process together forming a beam exiting the device.
In the present invention, the essential difference to the cyclotron is that RF switching occurs between multiple electrodes that are oriented radially with respect to a central electrode at the axis of ion rotation. Ions therefore gain circumferential velocity by firstly accelerating radially due to work done on them by those electric fields, then are subject to the magnetic cross-product force that transforms radial into circumferential momentum.
In the cyclotron the centripetal acceleration of the ion mass balances the magnetic force without regard to the radius and, as energy is gained, it freely increases its radius and so incrementally spirals outwards. Thus, it may appear counter-intuitive that for fixed-radius orbital stability a radial electric field must be configured such that it tends to accelerate ions away from the centre of rotation and oppose the magnetic field forces, but nonetheless is the case.
This configuration of electric fields is essentially that found in Penning traps used for ion storage. The present invention represents a planar complex projection of a Penning trap with the end cap electrodes translated into a central electrode, and with sychronised RF cycling more akin to a Paul trap. However, whereas the Penning trap has axial stability from the fields generated by the end caps, in the present invention all electrodes are in the same plane. Axial stability is therefore dealt with by other means in the present invention.
It will be explained later why an inward directed electric force results in an unstable system, but the outward electric force presents certain difficulties that are resolved by elements of other inventions. An orbiting ion bound within a radially outward electric force in combination with the magnetic force is self-stabilising for small perturbations about its orbit both spatially and temporally. However for larger perturbations, e.g. close nuclear scattering events, an ion disturbed from the control of the magnetic field will be strongly accelerated radially outward due to this imposed electric field. The energy of the ion would be lost from the circular beam if it were not recirculated back into the confines of the magnetic and electric fields once again.
Therefore, certain embodiments of the present invention also share commonality with a second set of inventions, designed for net fusion energy gain, from Philo Farnsworth (U.S. Pat. No. 3,386,883 and others). These devices operate by recirculation of positive ions back-and-forth through an electrode structure held at a negative potential to the surrounding potentials. These prior patents use spherical electrodes as a means for beam focussing. This is not the case for the present invention in which the outer electrodes are planar, but reciprocation of ions across the electrodes of the present invention, until such time as they are recovered into the circular beam, would improve the efficiency of the recovery of scattered ions. Further, in the present invention a final outer pair of concentric grids about the entire assembly may be employed to recirculate any other ions scattered at a sufficiently oblique angle that they pass the outer electrodes and would otherwise exit the device. Such structures in the present invention should therefore be as transparent to the passage of ions as practical, namely should be generally mesh or wire structures, for embodiments requiring maximum efficiency of operation.
A static radial electric field with crossed magnetic field will retain ions already in a stable, unperturbed, circular orbit. The present invention is principally based on the idea of time varying electric fields so as to keep and recover ions into a circular orbit even after perturbations. This requires phased electric fields synchronised with the ion orbit and therefore shares common ideas with rotating magnetic fields of electric motors, whereas the present invention uses rotating electric fields. George Meacham's invention, (US2005/0249324), also aims to accomplish net fusion energy gain by means of synchronised electric fields but this invention lacks a central electrode. In the present invention the central electrode forms a fixed rotation point for the ions but also provides the radial electric field which performs work on ions in the device thereby accelerating them to the speeds required to be held in the circular orbits.
Rolf Stenbaka's invention, (U.S. Pat. No. 4,853,173), aims for net fusion energy gain with an ion beam entering a storage ring composed of an outward directed radial electric field and orthogonal magnetic field, as per the present invention. This invention aims to collapse the storage ring to a central point by switching off the electric field. In the present invention ions are held permanently in a circular orbit by continuous electric field rotation, and fusion is intended between the fast beam ions and the background medium. Again, this has no central electrode so ions must be accelerated into this device by other means, namely an ion gun.
This configuration of radial electric field crossed by a magnetic field is therefore well-known, and there is extensive published research covering rotating plasmas in a EXB configuration. But the present invention rotates the whole electric field within a set of dissimilarly and periodically charged electrodes working in synchrony with rotating particles, rather than relying on the homopolar EXB principle that is common to most investigations into rotating plasmas to date.